1. Field of the Invention
The present invention relates to methods and systems for determining vehicle attitude, and particularly for determining vehicle attitude using a global positioning system (GPS).
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below at the end of the Detailed Description of the Preferred Embodiment. Each of these publications is incorporated by reference herein.)
The use of the global positioning system (GPS) is now widespread and it seems to have become a preferred instrument for high accuracy position and velocity estimation in inertial space. However, a stand-alone GPS receiver is unable to estimate the attitude of an aircraft, information critical to the pilot in adverse weather or at night. To obtain attitude information, the GPS is often coupled with an inertial instrument or a beacon [1]. However, it is well known that this coupled system needs nonzero acceleration to keep the aircraft yaw estimate from wandering off.
The current global positioning system is composed of a constellation of satellites. A GPS receiver is composed of an antenna and a receiver unit. The antenna receives position and velocity information from all satellites within a pre-determined cone. The information from each satellite is fused within the GPS receiver unit into a unique set of three positions and three velocities in the coordinate system called Earth-Centered-Earth-Fixed (ECEF). The information from the GPS has not been fused with the dynamics of the platform. In other words, the GPS information does not explicitly take into account whether the antenna is on a hand-held device, a parked car, a moving boat or a cruising aircraft; no additional information from the geometry or the dynamics of the platform is used.
To gain precision on the position and velocity information, multiple schemes have been developed in which the GPS sensor is aided by beacons or inertial systems like VOR, LORAN C or inertial measurement units (IMU). When the GPS is to be used as a stand-alone instrument for attitude determination, multiple antennas distributed over the platform are required. The main problem with using a single GPS antenna is the inability of the sensor to determine attitude.
To obtain attitude, the GPS has been previously coupled with an inertial instrument or has used multiple antennas. The result is an estimate of attitude at the cost of additional hardware and therefore weight and complexity in the navigation system. Moreover, the multiple antennas systems require the antennas be spaced about a meter apart. The cost of the alternative accurate GPS/inertial or multiple antennas navigation systems represents a considerable investment, say for general aviation aircraft and is mostly restricted to military use.
An apparatus and method for vehicle orientation determination is disclosed. A typical method comprises the steps of receiving position and velocity information data from a global positioning system receiver unit, receiving vehicle dynamics information data from one or more vehicle dynamics sensors and determining a vehicle attitude from the position and velocity information and the vehicle dynamics information data using a Kalman filter.
The present invention discloses a novel instrument capable of estimating accurately all aircraft states (position, attitude, velocity and angular rates) with only the input from a single low-grade inexpensive GPS receiver and the aid of the aircraft dynamics. This innovative navigation aid provides the aircraft attitude over the right envelope and in different exogenous weather conditions.